(1) Field of the Invention
The present invention relates to an apparatus and a method for improving image quality by detecting and filtering noise pixels to reduce image artifacts. The present invention relates more particularly to an image processing method for detecting and reducing ringing artifacts and an image processing apparatus that performs image processing according to the image processing method.
(2) Description of the Related Art
Most video data are stored and broadcasted in digitally compressed formats, and most compression standards, e.g., JPEG, MPEG-2/4, use block-based discrete cosine transformation (DCT) and quantization to reduce data amount. During the compression process, coarse quantization and coefficient truncation results in coding noise. Ringing artifacts (also referred to as ringing distortions and ringing noise), one kind of the coding noise, occur nearby high contrast edges in relatively low energy sections of images.
FIG. 1 is a diagram illustrating an example of ringing artifacts.
As illustrated in FIG. 1, for example, a picture which has been coded and then decoded shows a mountain, sky and a lake. In this case, an image which should show the mountain and the lake is represented as a texture with fine patterns. Ringing artifacts are noise that appears nearby edges of images showing objects such as mountains. Such ringing artifacts occur in decoded images because image coding removes a high-frequency component. To be more specific, ringing artifacts are made up of noise pixels scattered over blocks that include edge pixels. Due to the ringing artifacts, in blocks that include the mountain edge, for example, pixels which should show flat blue of the sky show a different color such as white, and pixels which should have pixel values as textures have different pixel values from the values as textures. As described, a picture which has been coded and then decoded has noise pixels that make up ringing artifacts.
Some methods for reducing ringing artifacts are proposed. An exemplary of reducing ringing artifacts is disclosed in a Patent Reference, U.S. Pat. No. 6,920,252 B2.
FIG. 2 is a diagram illustrating an image processing method disclosed in the above mentioned Patent Reference.
This conventional method includes a calculation step (ACT) of a spatial activity values, a determination step (NND) of a non-natural uniform area, a gradient filter step (GF), a detection step (RD) of a ringing artifact and a filter step (FIL). The conventional method determines ringing artifacts by comparing the activity and gradient of each pixel with a predefined threshold.
The activity of each pixel and its gradient are used to determine ringing artifacts in the conventional method. However, in natural images, textures of objects and scenes are similar to ringing artifacts. Textures and ringing artifacts usually have the same activity and gradient. By using the activity and the gradient, the conventional method cannot identify textures and ringing artifacts correctly. Once textures are mis-determined as ringing artifacts, the filter will remove those mis-determined textures. On the contrary, ringing artifacts are kept as textures when they are mis-determined as textures. Moreover, when textures and ringing artifacts are mixed in the same spatial locations, the error of pixel judgment makes the filter either remove ringing artifacts but lose texture or reserve texture but still keep ringing artifacts.
As described above, there is a problem with the conventional image processing method that it is impossible to detect ringing artifacts appropriately and reduce only the appropriately detected ringing artifacts.